This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A high pass filter is a very important component in a microwave communications system. A broad pass band, compact size and high power capacity are often required for a high pass filter. High pass filters constructed from quasi-lumped elements may be desirable for many applications, provided that these elements can achieve a good approximation of desired lumped elements over an entire operating frequency band. As known, a lumped element is designed so that electric or magnetic energy is concentrated in it at specified frequencies, and inductance or capacitance may therefore be regarded as concentrated in it, rather than distributed over the length of the element. For example, capacitors and inductors as shown in FIG. 1 are lumped elements. The actual dimension of a lumped element is much less than the operating wavelength. Compared to the lumped element, a quasi-lumped element is more frequency-dependent and behaves approximately as a lumped element as long as its maximum dimension is small compared with the operating wavelength.
Care should be taken when designing filters using quasi-lumped elements because as the size of any quasi-lumped element becomes comparable with the wavelength at an operating frequency, it no longer behaves as a lumped element.
The simplest form of a high pass filter may just consist of a series of capacitors, which is often found in applications for direct current (DC) blocking. For more selective high pass filters, more elements are required. This type of high pass filters can be easily designed based on a lumped-element low pass prototype as shown in FIG. 1(a), wherein g1 denotes the element value normalized by a terminating impedance Z0 and obtained at a low pass cutoff frequency Ωc. The cutoff frequency is a frequency characterizing a boundary between a passband and a stopband of a filter. It is usually taken to be a point in the filter response where a transition band and pass band meet, for example, as defined by a 3 dB corner (a frequency for which the output of the filter is −3 dB of the nominal passband value). If frequency mapping
  Ω  =      -                            ω          c                ⁢                  Ω          c                    ω      is applied, wherein Ω and ω are angular frequency variables of the low pass and high pass filters respectively, and ωc is the cutoff frequency of the high pass filter, any serial inductive element in the low pass prototype may be transformed to a capacitive element in the high pass filter, with a capacitance
      C    i    =            1                        Z          0                ⁢                  ω          c                ⁢                  ω          c                ⁢                  g          i                      .  Likewise, any shunt capacitive element in the low pass prototype may be transformed to a shunt inductive element in the high pass filter, with an inductance
      L    i    =                    Z        0                              ω          c                ⁢                  ω          c                ⁢                  g          i                      .  FIG. 1(b) illustrates such a lumped-element high pass filter resulting from the transformations.
Various transmission line structures have been applied to design high pass filters. Most of the filters implemented with this technique up to now are mainly based on such structures as stubs and quarter- or half-wavelength resonators, which may lead to a less compact size. In microwave and radio-frequency engineering, a stub is a length of transmission line or waveguide that is connected at one end only. The free end of the stub is either left open-circuit or short-circuited. Neglecting transmission line losses, the input impedance of the stub is purely reactive; either capacitive or inductive, depending on the electrical length of the stub, and on whether it is open or short circuit. Stubs may thus be considered to be frequency-dependent capacitors and frequency-dependent inductors. The quarter/half-wavelength resonator has a length of a quarter/half of the wavelength corresponding to the central frequency of the resonator. The lumped-element filter design is generally unpopular due to the difficulty for use at microwave frequencies along with limitations of lumped-element values. For a higher operating frequency, a lumped element with a smaller value is required. However, a lumped element with a too small value is difficult to fabricate.
Conventional microstrip filters lack sharpness at lower frequencies and suffer from larger insertion loss and poor impedance matching, specifically at high frequencies.
In particular, an existing solution disclosed in Reference document [1] uses a stripline structure to implement a high pass filter, which however has some problems, such as some stubs are difficult to fabricate; electrical lengths of connecting lines are relatively long; and tuning screws have to be used to reach required performance. Another existing solution disclosed in Reference document [2] uses a suspended stripline to implement an ultra-wideband filter. However, this solution has a low power capacity and the suspended line has to be fabricated by using a laser beam to meet a certain manufacturing accuracy and power capacity. Yet another existing solution disclosed in Reference document [3] uses a microstrip line to implement a bandpass filter. However, due to the microstrip line structure and low power capacity, the pass band of such a filter is relatively narrow and thus unsatisfactory.